A synchronous mesh-type transmission shown in FIG. 6 is known as an example of a transmission. In the transmission, a main shaft 5 and a countershaft (not shown) disposed in parallel with a predetermined distance therebetween are held in a mission case (not shown) such as to rotate freely. The main shaft 5 interlocks with an output shaft (driving wheel side). The countershaft interlocks with an input shaft (engine side).
A countershaft gear 6 is integrally (or separately) provided on the countershaft. A main shaft gear 1 is mounted on the main shaft 5 with a tapered roller bearing A therebetween, such as to rotate freely. A teeth section 1a that constantly meshes with the countershaft gear 6 is integrally provided in a center portion of the outer circumferential surface of the main shaft gear 1. A clutch gear 7 is engaged in an interlocking manner with both end portions of the main shaft gear 1. The clutch gear 7 has spline teeth 7a on the outer circumference and an integrally formed conical cone 7b on one side. A synchro-mechanism 8 is disposed adjacent to the clutch gear 7.
The synchro-mechanism 8 includes a sleeve 81, a synchronizer key 82, a hub 83, a synchronizer ring 84, a pressing pin 85, and a spring 86. The sleeve 81 moves in an axial direction (left-right direction in FIG. 6) as a result of a selector being operated (not shown). The synchronizer key 82 is mounted on the inner circumference of the sleeve 81 such as to move freely in the axial direction. The hub 83 is engaged in an interlocking manner with the outer circumference of the main shaft 5. The synchronizer ring 84 is mounted on the outer circumference of the cone 7b of the clutch gear 7 such as to slide freely. The pressing pin 85 and the spring 86 elastically press the synchronizer key 82 to the inner circumference of the sleeve 81.
In a state shown in FIG. 6, the pressing pin 85 holds the sleeve 81 and the synchronizer key 82 in a neutral position. At this time, the main shaft gear 1 receives the rotation of the countershaft gear 6 and spins freely around the main shaft 5. On the other hand, when the sleeve 81 moves, for example, to the left in the axial direction from the state shown in FIG. 6 as a result of the selector being operated, the synchronizer key 82 moves to the left in the axial direction so as to follow the sleeve 81 and presses the synchronizer ring 84 against the angled surface of the cone 7b of the clutch gear 7. As a result, the rotation speed on the clutch gear 7 side decreases while the rotation speed on the synchro-mechanism 8 side increases.
When both rotation speeds become synchronized, the sleeve 81 moves further to the left in the axial direction and meshes with the spline teeth 7a of the clutch gear 7. The main shaft gear 1 and the main shaft 5 become interlocked via the synchro-mechanism 8. As a result, the rotation of the countershaft gear 6 is decelerated by the main shaft gear 1 at a predetermined transmission gear ratio, and the rotation is transmitted to the main shaft 5. At this time, the main shaft gear 1 rotates synchronously with the main shaft 5 and a bearing inner ring 2 of the tapered roller bearing A.
The tapered roller bearing A used in a main shaft gear mechanism of a synchronous mesh-type transmission of an automobile is configured by the main shaft gear 1 that also serves as a bearing outer ring, a pair of bearing inner rings 2, double-row tapered rollers 3, and a pair of cages 4. The bearing inner rings 2 have a raceway surface 2a on the outer circumferential surface and are fitted onto the outer circumference of the main shaft 5. The double-row tapered rollers 3 are disposed between double-row raceway surfaces 1c of the main shaft gear 1 and the raceway surfaces 2a of the pair of bearing inner rings 2. The cages 4 each hold a row of tapered rollers 3.
During the above-described gear shift, as a result of the synchronized rotations of the main shaft gear 1 and the bearing inner ring 2, the rollers 3 serving as rolling elements stop on the raceway surfaces 1c and 2a. On the other hand, when vibrations and the like from an external source are repeatedly applied, repeated minute sliding occurs between the rollers 3 and the raceway surfaces 1c and 2a. A phenomenon referred to as fretting, in which a contact surface becomes worn as a result of repeated relative, minute sliding, may become a problem.
To suppress the above-described fretting, a parkerization process (phosphate coating process) may be performed on the main shaft gear, the raceway surfaces of the bearing inner rings, and the rollers to reduce frictional resistance between the rollers and the raceway surfaces. However, the parkerized coating may deteriorate. A favorable, long-term fretting suppression effect cannot be expected.
Conventionally, an invention (Patent Document 1) is known that includes an unbalancing means and the like actualized by circumferentially uneven placement of the tapered rollers, unbalanced weight in the circumferential direction of the cages holding the tapered rollers, and unequal weights of the tapered rollers. In other words, as a result of the center of gravity of the cages being shifted from the center of rotation, relative rotation can be achieved from a stopped state through use of moment of inertia.
Patent Document 1: Japanese Patent Laid-open Publication No. 2000-193069